Ceramics-Silikáty 65, (4) 368 - 376 (2021)

The optimisation of alumina foam slurries for fast freeze-drying was determined and the final properties of the porous specimens were examined. The slurry solid loadings were varied from 50, 60, and 70 wt%. The impact of a foaming agent (a naturally modified anionic surfactant), which generated bubbles, was investigated comparing samples without the foaming agent and with a 5 wt% foaming agent. The amount of binder (polyvinyl alcohol) was varied from 5 and 10 wt%. It was found that increasing the slurry solid loadings and the binder content resulted in a slower freeze-drying. This was due to the difficulty in breaking the bonds between ice crystals and the binder molecules during sublimation. Also, the high solid loadings decreased the porosities and narrowed the pore channels causing ice crystals to be trapped, which slowed down the process. A foaming agent addition could shorten the freeze-drying duration up to 25% when the binder was present with no more than 5 wt%. Therefore, the optimised formula found in this study was B5F5_60, which showed a porosity of 76% and a strength of 4 MPa. This indicates that a highly porous material can be achieved with a fast freeze-drying technique.